1. Field of the Invention
The present invention generally relates to a semiconductor laser device. More specifically, the present invention is related to a semiconductor laser device having high output power/high efficiency characteristics, and also superior single-mode characteristics.
2. Description of the Related Art
As a method for optically coupling a semiconductor optical waveguide device (namely, one optical function device) to a single-mode optical fiber (namely, the other optical function device), a semiconductor laser diode and a semiconductor switch are employed. In this xe2x80x9cButt-couplingxe2x80x9d method, an edge surface of an optical waveguide device directly abuts against an optical fiber.
However, in accordance with this xe2x80x9cButt-couplingxe2x80x9d method, since the spot sizes of the laser light propagated through the optical waveguides are different from each other, such a coupling loss indicative of the loss in the waveguide light amount would occur at the direct abutting portion.
Normally, a spot size (mode diameter) of a laser beam projected from a semiconductor device is selected to be on the order of 1 micrometer. Also, normally, a spot size of a laser beam for an optical fiber is selected to be approximately 5 micrometers. As a result, a coupling loss occurred in the xe2x80x9cButt-couplingxe2x80x9d between the semiconductor optical device and the optical fiber will become approximately 10 dB.
Conventionally, as a method for reducing a coupling loss, a lens is employed for converting spot sizes of laser beams.
However, as to the coupling method with using such a lens, since tolerance in the lens alignment is small, it is practically difficult to assemble this lens with the semiconductor optical device. As a consequence, manufacturing cost of the respective modules would be increased.
In particular, very recently, application fields of semiconductor laser diodes are rapidly extended from ground transmission systems to other systems such as a subscriber system, a LAN (local area network) system, and a data link system. Under such a circumstance, a large number of semiconductor laser diode modules manufactured in low cost are necessarily required. As a consequence, total numbers of components required when one semiconductor laser diode itself is manufactured are desirably reduced. Furthermore, since the major portion of the manufacturing cost of these modules is caused by the difficult assembling work, the optical fiber is coupled with the semiconductor laser diode by employing an easy assembly by a passive alignment.
Based upon the above-described technical aspects, various sorts of optical coupling devices and various types of light sources in which optical coupling devices are integrated with semiconductor laser diodes have been developed.
For instance, the beam spot-size expanded laser diode with the laterally tapered active stripe is described in xe2x80x9cInternational Conference on Indium Phosphide and Related Material, Conference Proceedingsxe2x80x9d pages 657 to 660, in 1997. In this beam spot-size expanded laser diode, the entire resonator is composed of the active layer region. And this beam spot-size expanded laser diode can oscillate laser light with the superior temperature characteristic and also the high convergence rate.
FIG. 1A illustratively shows the structure of the above-described beam spot-size expanded laser diode with the lateral tapered active stripe.
Referring now to FIG. 1A, the beam spot-size expanded laser diode has a following-described structure.
A P type InP layer 721 and an InGaAsP active layer 701 are formed on a P type InP substrate 708.
An N type InP layer 714 is formed on a P type InP layer 721.
A P type InP layer 712 is formed on an N type InP layer 714.
An N type InP layer 710 is formed on the entire surface of the element.
Furthermore, a film 703 having a low reflective power is provided on a front surface of the beam spot-size expanded laser diode.
Also, a film 702 having a high reflective power is provided on a rear surface of the beam spot-size expanded laser diode.
FIG. 1B illustratively represents the shape of the active layer 701 contained in this beam spot-size expanded laser diode with the lateral tapered active stripe.
The active layer 701 includes an active layer region 705 and a tapered active layer region 706. The tapered active layer region 706 has such a structure the width of the tapered active layer region 706 is narrowed at a region near the laser light radiation plane.
Referring now to FIG. 1A and FIG. 1B, this beam spot-size expanded laser diode with the lateral tapered active stripe owns such a structure that the width of the active layer 701 is narrowed at a region near the laser light radiation plane.
In this semiconductor laser diode, since the entire resonator corresponds to the active region, it is possible to relatively shorten the length of this semiconductor laser. As a result, this semiconductor laser diode owns an advantageous structure to have a high convergence rate. Also, this specific semiconductor laser diode may be manufactured by utilizing the same manufacturing steps for the semiconductor laser diode having the conventional uniform active layer width.
In the beam spot-size expanded laser diode with the laterally tapered active stripe, when the length of the tapered active layer region 706 is shortened while maintaining the length of the resonator at a constant value, the volume of the active layer 701 is increased. As a result, the gain of this semiconductor laser diode is increased to reduce the threshold current value and the operating current under high temperatures. Also, the shorter the length of this tapered active layer region 706 becomes, the higher the coupling degree to the radiation mode is increased to reduce the coupling efficiency of this semiconductor laser diode with respect to the optical fiber.
Other conventional semiconductor laser devices have been described as follows.
Japanese Laid-open Patent Application (JP-A-Heisei 10-22577) describes xe2x80x9cLIGHT EMITTING SEMICONDUCTOR DEVICExe2x80x9d. This light emitting semiconductor (laser diode) device is equipped with the beam spot-size conversion structure, and is capable of blocking such a phenomenon that the laser light leaked out from the core in the spot-size conversion region is returned to the gain region of this laser diode. Concretely speaking, this laser diode is featured by having the reflection structure capable of reflecting the scattered laser light in the vicinity of the spot-size conversion structure and the gain region. This scattered laser light is leaked out from the core and then is entered into the gain region.
Also, Japanese Laid-open Patent Application (JP-A-Heisei 9-61652) discloses xe2x80x9cSEMICONDUCTOR OPTICAL WAVEGUIDE AND MANUFACTURING METHOD THEREOFxe2x80x9d. In the semiconductor optical waveguide and the manufacturing method thereof, when the optical element such as the semiconductor laser device is coupled to another optical element, or the optical fiber without using the lens system, the coupling efficiency can be increased. The structure of this semiconductor optical waveguide represents such a tapered shape that the thickness of the core layer of the optical waveguide is continuously changed in an exponential manner.
The present invention has been made to solve the above-explained problems.
Therefore, an object of the present invention is to provide a semiconductor laser device capable of realizing a high coupling efficiency between this semiconductor laser device and an optical fiber under a condition that this semiconductor laser device has a tapered structure of a short length.
Another object of the present invention is to provide a semiconductor laser device capable of achieving a low threshold value and a high coupling efficiency at the same time under high temperature.
In order to achieve an aspect of the present invention, a semiconductor laser device includes an electron carrying layer, an active layer, and a hole carrying layer. The electron carrying layer is formed on a semiconductor substrate. The active layer includes a first straight active layer region having a first width and a second straight active layer region having a second width. And the active layer is formed on the electron carrying layer in contact with the active layer. Here, the first straight active layer region is joined to the second straight active layer region, the second width is narrower than the first width, and an active layer radiates laser light in response to an application of a voltage higher than or equal to a predetermined voltage. A hole carrying layer is formed on the active layer in contact with the active layer.
Both of the first width and the second width may be such that the laser light in a waveguide mode interferes with the laser light in a radiation mode in the second straight active layer region.
Both straight active layer regions have only lowest guided mode.
The active layer may radiate laser light of a wavelength of xcex micrometer, the first width of the first straight active layer region may be substantially 0.923xc3x97xcex micrometer, and second width of the second active layer may be substantially 0.462xc3x97xcex micrometer.
The active layer may radiate laser light of substantially a same wavelength as 1.3 micrometer, the first straight active layer region may have the first width of substantially a same width as 1.2 micrometer, and the second active layer may have the second width of substantially a same width 0.6 micrometer.
The second active region may have a length such that a phase of the laser light in a waveguide mode is equal to a phase of the laser light in a radiation mode in the active region.
The semiconductor laser device may further include a current blocking layer formed between the electron carrying layer and the hole carrying layer.
The current blocking layer may comprise a hole blocking layer and an electron blocking layer. Here, the hole blocking layer is formed on the electron carrying layer and the electron blocking layer is formed on the hole blocking layer.
The semiconductor laser device may be coupled to a single-mode optical fiber and a coupling efficiency between the semiconductor laser device and the single-mode optical fiber is lower than, or equal to 5.0 dB.
The active layer may have a length such that a loss of the laser light propagated is smaller than or equal to 0.3 dB. Here, the loss occurs while the laser light once goes to and returns the active layer.
The semiconductor laser device may further include a diffraction grating formed under the second active region.
The active layer may radiate laser light of a wavelength of xcex micrometer and the diffraction grating may have substantially a interval of xcex/6.364 micrometer.
The active layer may radiate laser light of substantially a same wavelength of 1.3 micrometer and the diffraction grating may have substantially a same interval of 204.3 nanometer.
The semiconductor laser device may be operated in a single mode such that a sub mode suppression ratio is lower than or equal to 50 dB.